What causes emails to end up in spam?

Question

gmail

The bottom line is the following, raised a small mail server for information mailing letters to customers. Made all necessary settings and added all SPF, DKIM, DMARC records. I check - all mailers except gmail perfectly receive letters in the inbox. Gmail constantly puts emails in spam, despite the fact that all signatures pass verification.
I tried to fight this for a long time and at one point I tried to send a letter not through the mail web interface, but through my own script that makes the mailing list. And, lo and behold, not a single gmail message was put in spam, and even marked as important! All this is due to the fact that the script that makes the mailing list explicitly indicates the name of the recipient and gmail determines that my script knows exactly who it sends the letter to, a kind of check for gmail that the subscriber has subscribed and indicated his data, thereby giving your consent to receive emails from me. But what if you still need to send a cold letter, or the client did not specify his name or the name is different from the one indicated in his gmail? What else does gmail pay attention to when receiving an email?

Answer 1

What else does gmail pay attention to when receiving an email?

Where the letter was sent from (program, script) and the presence of links.
From practice, I will say that a large number of identical letters should not be sent. 50 messages and change the text.